Fast metabolism is one of key issues for many drug candidates, which could result in poor bioavailability, short half-life and significant drug-drug interactions.To identify the issue in early phase of drug discovery, lead compounds are routinely screened by human liver microsome (HLM) metabolic stability assay.Compounds that have poor metabolic stability are either discarded or subjected to metabolic soft-spot analysis.Results from the metabolic stability and soft-spot analyses can facilitate the design of metabolically stable compounds via structural modification.Metabolic soft-spot assay is often conducted in HLM incubations followed by quantitative and qualitative profiling of metabolites by LC-MS.